The JWST Spectroscopic Properties of Galaxies at $z=9-14$

Abstract

We characterize the JWST spectra of $61$ galaxies at $z=9-14$, including $30$ newly-confirmed galaxies. We directly compare the $z>9$ spectroscopic properties against $401$ galaxies at $6<z<9$, with the goal of identifying evolution in the star formation histories and ISM. We measure rest-UV emission line properties and UV continuum slopes, while also investigating the rest-optical emission lines for the subset of galaxies at $9.0<z<9.6$. With these spectra, we constrain the stellar masses, specific star formation rates, dust attenuation, and the average metallicity and abundance pattern of $z>9$ galaxies. Our dataset indicates that the emission lines undergo a marked change at $z>9$, with extremely large CIII], H${\hat{I}}^2$, and H${\hat{I}}^3$ EWs becoming $2-3\times$ more common at $z>9$ relative to $6<z<9$. Using the spectra, we infer the distribution of SFRs on short (SFR${}_{3Myr}$) and medium (SFR${}_{3-50Myr}$) timescales, finding that rapid SFR upturns (large SFR${}_{3Myr}$/SFR${}_{3-50Myr}$ ratios) are significantly more likely among $z>9$ galaxies. These results may reflect a larger dispersion in UV luminosity at fixed halo mass and larger baryon accretion rates at $z>9$, although other physical effects may also contribute. We suggest that the shift in star formation conditions explains the prevalence of extreme nebular spectra that have been detected at $z>9$, with hard ionizing sources and nitrogen-enhancements becoming more typical at the highest redshifts. Finally, we identify five $z>9$ spectroscopically confirmed galaxies with red UV colors (${\hat{I}}^2\gtrsim -1.5$), either revealing a small population with moderate dust attenuation ($\ddot{I}{„}_V=0.23-0.35$) or very high density nebular-dominated galaxies with hot stellar populations.

39 pages, 15 figures, accepted by ApJ